Centrifuge



Dec. 24, 1935.

Patented Dec. 24, 1935 UNITED STATES PATENT OFFICE CENTRIFUGE Walter F.Hoflman and Roy P. Hello, Cloqnet, Minn., assiznors to Ccllovis, Inc.,Chicago, Ill., a corporation of Delaware Application March 28, 1932,Serial No. 601,566

Claims.

The present invention relates to reactive treatments of cellulose. Ithas particular reference to the esterification by nitration of shortfibers of cellulose, such as wood fibers. Although wood fibers areclassified generally into long fibers and short fibers, we do not referin this invention to these subdivisions of wood fibers, but aim todistinguish between the fibers of natural cotton cellulose and thefibers of chemically prepared wood cellulose.

Cotton is the common form of cellulose which is nitrated, and its fibersare distinctive. Cotton fiber, such as linters, is nitrated with amixture of sulphuric acid and nitric acid. Excess acid and the resultanttreated fibers are quickly separated in a centrifuge. Although thechemistry of the process is or may be the same for nitrating suitablechemical wood pulp, which also is cellulose, it has been found there aremany dlfliculties encountered by the mere substitution of wood pulpfibers for cotton fibers. The present invention aims to overcome thesedifiiculties.

Cotton by nature easily mats into a. fiufl or ball, or whatever size orform of mat is permitted, in such a way that the mat is highly porousand absorbent. For the purpose of this invention it may be described aspresenting open channels to its interior, so that esterifying acid maypenetrate to the interior as a liquid flowing in the channel, asdistinguished from diffusion of liquid through a mass of more compressedfibers or through capillary channels between or within fibers. Inesterification, especially in nitration, particularly where the processis carried out in a relatively short time, it is essential that all thefibers be esterified and be uniformly esterified. Hence it is essentialthat the interior of fibrous balls or mats be quickly reached by acid.Where the exterlor fibers of any such mass are effective to bar aflowing access of acid to the interior, the process of transfer throughthe obstructing exterior fibers may wholly or partly exhaust the activestrength of the acid, and an incompletely esterifled or an unesterifiedproduct is obtained on the interior. Although a large part, such as 95%,may be properly esterified, a slight residue of even slightlyunder-treated fibers, will manifest itself as undissolved or partiallydissolved fibers, or as cloudiness in a solution of the mass. Because itis difflcult to remove fibers or cloudiness from such solutions, it isimportant that uniform esterification be practiced.

Another difilculty experienced is the separation of fibers and spentacid after the esterification process is completed. For cotton, aperforated basket may be employed in the centrifuge. This is asufficient screen to quickly separate cotton fibers and nitrating acid,without significant loss of fiber, and without undue retention of acid.5.. When chemical wood pulp, such as that prepared by shredding driedlap board, is used in the same type of centrifuge, there is anappreciable and economic loss of fiber through the perforations. Thereis also matting of the fiber into a sheet, or filter pad, over the holesin the basket, making it a very slow and difiicult process to effectseparation. Prolonged contact of the acid, even after the esterifyingprocess is technically complete, may injure the product. It is otherwiseimportant to hasten the separation because, as nitration is practicedcommercially, one centrifuge. may be used in rotation with severalnitrating pots.

In attempting to overcome the clogging of cen- 2o trifuges for nitrationmasses comprising concentrated acid mixtures and suspended fibers ofesterified cellulose, numerous schemes have been tested. In thepaper-making industry large masses of fiber, such as those blown from adi- 5 gester, are drained of waste liquor, and washed by percolation, i.e., by providing drainer bottoms of perforated plates or tile. Aconsiderable depth of liquor say over 10 feet, causes a high pressure onsuch drainer bottoms comparable to pressures in a centrifuge in action.While such perforated bottoms are suitable for the filtration ofchemical wood pulp and waste cooking liquor, it has been found that theyare not suitable for nitrated Wood pulps suspended in waste nitratingacid. Perforated metal plates with specially designed perforations,based onpast experience with pulp filtrations, were found to beunsatisfactory.

After numerous and various attempts to provide a suitable centrifugebasket it was discovered that the fibers from wood, compared to cottonlinters, caused such a tight packing over perforated metal walls 02'sheets that filtration of acid through the pads over the holes furthercompacted the pads. Any dirt or residue in the nature of filterablematerial other than fiber, readily clogs the pad after it is formed.Prior to and during formation of the pad, many short fibers pass throughthe openings. By making the perforations smaller and more numerous,packing and clogging is increased. The fibers not only pack over theholes, but pack tightly against the metal walls between the holes sothat no channel for flow exists at the interface of the fiber and thenon-perforated areas of the wall. v

By providing a fine wire screen between the nitrated fibers and theperforated wall of the ordinary centrifuge, we have been able toovercome all the difficulties above enumerated; The wire screen providesa uniform and a maximum filtering area so that nitrated fibers packuniformly and uniformly resist flow therethrough, as distinguished fromlocal packing and clogging with perforations. A uniform density of matresults, providing a uniform filtering area over the entire basket. Theface of the screen against the basket wall provides channels for fiow offiltered acid along the walls to outlets, such as perforation, in thewalls.

The effectiveness is due to the screen, and is not due to the perforatedcharacter of the centrifuge basket described. The fact that the basketis perforated is immaterial. Other means might be provided forpermitting acid to escape after it has passed through the screen. Theperforated basket is merelyexemplary of means rigidly to support thescreen'in a nonyielding position to form a balanced basket. It is thepreferred means for the reason that ordinary nitrating centrifuges ofcommerce, and of accepted design, may be used for wood pulp by theaddition of a screen lining on the interior thereof. Such screens areeasily replaced without removal of the basket from its customaryhousing.

In using a screen, a metal wire screen of the so-called non-corrosive oracid-proof metals may be employed, although there is no need to limitthe screen to such materlal. i-Any material which has a life in the acidencountered is suitable, which life is commensurate with initial andreplacement costs. I have successfully employed nickelchromium alloywhich is 60% nickel, 13% chr0- mium, 25% iron, and 2% manganese.

A basket weave wire cloth has been found most suitable. Such a cloth maybe made of 14 warp strands per inch, each .014 inches in diameter. Theshute or filling wires may be .011 inches in diameter, thus giving about100- meshes per inch. Of course, the sizes of the wires and the spacingof the warp may be varied, and this may be determined somewhat by thecharacter, such as stiffness, length and diameter, of the fiber usedwith the screen. In the centrifuge it is preferably used with the warpwires lying circumferentially.

Thus, the shute wires extend parallel with the axis of the basket andact as a grid over which the fibers must slide as there is slippage inrotation. A fiber moving endwise to the wall can thus have its endcaught by a shute wire, which will result in fastening it so that itwill be dragged, and its body will lie across the shute wires andparallel to the warp. With the wire arranged with the warp parallel tothe axis, the elongated openings would be arcuate about the axis, andpwsage therethrough will be easier than where the elongation is directedas described. Where the basket is truly-cylindrical as shown, or whereit may be another form of revolution (geometrically speaking) the shutewires are substantially geometrical .elements of the circular screenwall. The direction of the elongations of the openings are thus angular,and in a cylinder at right angles,-to the plane of rotation.

Aspen wood pulp is one of the shortest of ordinary chemical wood fibersand we have found that the screen described is satisfactory forretaining and for filtering waste nitrating acid from nitrated aspenfibers. In such nitration we have used the process described in theapplication of W. C. Wilson, Serial No.119,995, filed July 1, 1926,which is a short-time nitration, where rapid removal of the acid fromthe pulp at the end of the nitrating reaction is desirable. Efiicientremoval is effected by the present invention so that little acid iswasted when the pulp product is removed and drowned in water. It isusually desirable to make a quick transfer of the whizzed fibers towater, and the wire screen of the present invention, when used herein asdescribed, isin no way a detriment to the quick emptying of thecentrifuge.

Fig. 1 represents a vertical cross-section of a ctaiammon centrifugeembodying the present invenon. 15 Fig. 2'is a detailed view of thepreferred screen construction.

Fig. 3 is a fragmentary plan view of the top of the centrifuge basketshowing one method for securing the screen. g

The numeral l0 represents a type of centrifuge basket used in nitratingcotton. It has metal wall ll supported by numerous rings designated i2.The wall may be perforatedwith inch holes 13 on 1 inch centers fornitrating cotton. Although such a basket may be used for centrifugingnitrated cotton, it is subject to the disadvantages above mentioned whenused for separating nitrated wood pulp from nitrating acid. However, byplacing a screen, such as resistant wire cloth of fine mesh, within thewall as a lining, the basket may be adapted form with nitrated woodpulp. vNumerousmechanical expedients may be resorted to to secure thescreen in the basket.

A strip ll of woven wire cloth such as that above described may be laidas a lining inside the basket. The ends I! may be riveted together as atI! or merely allowed to overlap. The edge is allowed to project abovethe basket 4 and is cut at intervals I! to the edge of the basket. Thetabs l9 thus formed are bent over the top against the upper ring I2. Anannular cover plate 20 is screwed at 2| to the ring l2 and this securesthe screen in place. The cover plate projects inwardly of the basket andprovides a smaller circular opening 22. Where a basket is 30 inches indiameter the opening 22 may be 20 inches in diameter. Such a basket maybe 16 or 1''! inches deep. However, these dimensions are notcontrolling, being herein given as illustrative. The warp wires 23 areshown running circumferentially about the basket, and the shute wires 24are shown at right angles thereto lying adjacent each other in piledformation. In Fig. 2 the holes are behind wires and cannot be seen.

The woven wire cloth having a basket weave is an example of ascreen inwhich there are no perforations normal to the screen surface. Thecharacteristic perforation is tortuous, requiring for direct normalpassage through the screen, as occurs in a centrifuge lined with such ascreen,

a devious path around a wire, and otherwise requiring for a straightpassage therethrough one 5 which is angular to normal. Without limitingthis invention to any particular theory in connection with thesuccess-of this wire cloth for wood pulp fibers, it is believed thatthere is relation of the character. of opening in a centrifuge to thecharacter of fiber.

By the term basket-woven as applied to wire or cloth it is intended toindicate that the warp strands are spaced and that the shute or fillerstrands are piled and are adjacent to each other 7 and cross in contactwith each other in the spaces between the warp strands.

The mass of openings provided by such a basket-woven wire cloth asillustrated is in a checkered-like arrangement, and the communicationinto the wall of the centrifuge from the interior of the basket isresultingly at an angle to the radius of the plane of rotation, and isdirectly blocked in the line of said radius.

Cotton fiber is in diameter larger, stiffer, and more kinky thanchemical wood fibers. The latter are more straight as they grow innature, finer, and after chemical liberation, are more flexible. Inflowing in a streaming suspending liquid, they tend to move endwise instraightened form to a greater extent than cotton fibers. This enables amass of them to flow through openings in a centrifuge which ordinarilywill retain a mass of cotton fibers. Particular note should be made thatthis invention deals with the properties of a mass of fibers, ratherthan the properties of an individual fiber, that the nitrated fibers,rather than the original fibers are concerned, and that a waste acidmixture is the suspending medium, and not water. It is possible thatseveral fibers jam together in a perforation and form a pad. This wouldbe less likely to occur in a punched or drilled holein a basket or in asheet; The small wire obstructions between openings no doubt aid incatching and holding fibers looped about the wire.

Although the'improvement appears simple, it is marked in results and hasshown itself an important factor in economy, that is, in preventing lossof material and loss of time; that the adoption has caused success,where impractical results were obtained in devices used for cottonlinters.

It is pointed out that heretofore in the use of chemical wood pulp fornitrations, free fibers have not been employed. It has been a practiceto use tissue sheets, torn or cut into small pieces, which maintain thefelted character during nitration and in separation do not exhibit incentrifugal separation the difiiculties here described when nitratedchemical wood pulp in free fibrous form is suspended in the residualacid of nitration.

In the accompanying claims we aim to include such modifications anddepartures from the invention as will appear to those skilled in theart, and which fall within the scope of the appended claims.

We claim:

l. A centrifuge comprising a rotary basket having a circular wall ofbasket woven wire cloth arranged with the shute or filling wires lyingsubstantially in planes passing through the axis of rotation of thebasket, said cloth having spaced warp strands extending substantiallycircumferentially of the basket in the 5 plane of rotation and havingshute strands lying in contact with each other in piled formation andwoven alternately above and below the warp strands.

2. In a centrifuge a basket having numerous 10 minute perforations inthe wall and a wall surface presenting a fine grid structure extendinggenerally in the direction of the axis, and means crossing each openingin the grid structure forming elongated axially directed grid openings15' whereby suspended fibers moving endwise in liquid in the basketduring rotation may be caught at the. end by said grid, and be turned tolie across the grid.

3. A centrifuge comprising a rotary basket 20 having a circular wall ofbasket woven wire cloth arranged with the shute or filling wires lyingat a substantial angle to and generally cross-wise of the plane ofrotation of the basket, said cloth having spaced warp strands extendingsubstan- 25 tially circumferentially of the basket in the plane ofrotation and having shute strands lying in contact with each other inpiled formation and woven alternately above and below'the warp strands.

4. A centrifuge comprising a rotary basket having a circular wallprovided on its inside surface with a mass of small elongatedperforations arranged in checkered-like positions over substantially theentire area of the filtering zone :5 of said wall, the direction ofelongation being angular to and generally crosswise of the plane ofrotation, said openings providing communication into said wall surfaceonly at an angle to a radius in said plane of rotation.

5. In a centrifuge a basket having means permitting the escape of liquidfrom the basket in centrifugal action of the basket, and filtering meansproviding an inner filtering surface having a fine grid structureextending generally in 4.5 the direction of the axis and having meanscrossing each opening in the grid structure forming elongated axiallydirected grid openings, whereby suspended fibers moving endwise inliquid in the basket during rotation may be caught by 50 said grid andbe turned to lie across the grid, said liquid passing through saidfiltering means for escapexnent from the basket.

WALTER F. HOFFMAN. 55 ROY P. BELLA.

